Internal combustion engines may utilize direct fuel injection, wherein fuel is directly injected in to an engine cylinder to improve mixture preparation and to reduce cylinder charge temperatures. An amount of time a direct fuel injector is activated may be a function of fuel pressure supplied to the injector, engine speed and engine load. Therefore, at higher pressures, a fuel pulse width supplied to the injector may be adjusted to a short duration of time (e.g. less than 500 micro-seconds).
However, operating the fuel injector with short pulse widths may cause the injector to operate in a non-linear or ballistic region where the amount of fuel injected may vary substantially for small changes in the fuel pulse width. For example, the direct fuel injector may deliver less fuel than desired in the ballistic region where shorter pulse widths are applied to the fuel injector. Further, the variability in the ballistic region may not show a linear trend. Also, fuel injectors delivering fuel to the cylinder often have piece-to-piece and time-to-time variability, due to imperfect manufacturing processes and/or injector aging (e.g., clogging), for example. Consequently, injector variability may cause cylinder torque output imbalance due to the different amount of fuel injected into each cylinder, and may also cause higher tail pipe emission and reduced fuel economy due to an inability to correctly meter the fuel to be injected into each cylinder.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for a cylinder, comprising: during a learning condition, delivering a first pulse width and a second pulse width to a fuel injector supplying fuel to the cylinder during a cylinder cycle; varying a ratio of the first pulse width to the second pulse width; and determining an injector variability transfer function based on the ratio and an engine lambda value; and adjusting a control parameter of the injector based on the transfer function.
By supplying two pulse widths to a fuel injector during a cycle of a cylinder receiving fuel from the fuel injector, it may be possible to provide the technical result of adjusting a fuel injector transfer function or gain without having to operate the cylinder with an air-fuel ratio that may be leaner or richer than is desired. In particular, a first pulse width supplied to a fuel injector may be adjusted to be short enough in duration to operate the fuel injector in its non-linear low flow region. A second pulse width supplied to the fuel injector during a same cylinder cycle may be long enough to operate the fuel injector in its linear operating range so that a fuel amount closer to a desired fuel amount may be supplied to the cylinder during the cylinder cycle. Consequently, if fuel supplied by the fuel injector in response to the first pulse width is greater or less than a desired amount, the aggregate air-fuel mixture during the cylinder cycle may be less affected because a greater amount of a desired fuel amount to be injected to the cylinder may be provided via the second pulse width operating the fuel injector.
Further, a ratio of the first pulse width to the second pulse width (also referred to herein as a split-ratio) may be varied by decreasing the first pulse width and increasing the second pulse width. As the ratio is varied, a relative change in an engine lambda value from nominal may be measured. Due to injector variability resulting in significant errors in delivering fuel in the low pulse width operating regions, the relative change in the lambda value corresponds to the decrease in the first pulse width. Therefore, the relative change in the lambda value may be utilized to determine a correction factor which may be applied to a fuel injector transfer function. In this way, by sweeping the split-ratio and determining the change in lambda value from nominal, injector variability may be learned and applied to obtain a more accurate fuel injector transfer function.
The present description may provide several advantages. In particular, the approach may reduce engine air-fuel errors. Additionally, the approach may allow a fuel injector to be operated at pulse widths that were heretofore avoided because of non-linear fuel injector behavior. As such, this improves the operating range of the fuel injector. Further, the approach may reduce engine emissions and improve catalyst efficiency.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.